1. Field of the Invention
The present invention relates to an ink jet recording device including a plurality of head modules for forming a high quality image at a high printing speed.
2. Related Art
A conventional serial-type ink jet recording device repeatedly performs a scanning operation and a feeding operation in alternation for forming an ink image on an elongated uncut recording sheet. Specifically, a print head of the recording device forms, in the scanning operation, a single-scan-worth of image on the recording sheet while scanning in a main scanning direction, which is a widthwise direction of the recording sheet. Then, the recording sheet is fed, in the feeding operation, in a secondary scanning direction perpendicular to the main scanning direction by a predetermined amount. After repetition of these operations, a whole image is completed.
In order to improve the printing speed of this type of ink jet recording device, there has been proposed to increase the amount of the image that the print head can form in a single scan. There has been also proposed to form the print head by assembling a plurality of short head modules in order to provide the print head with an elongated width.
When assembling the plurality of short head modules, the preciseness in assembling is important in producing a high quality image. However, it is hard to achieve the sufficiently precise assembling. In order to overcome this problem, Japanese Patent-Application Publication No. HEI-5-305734 proposes a method for adjusting the positional relationship among the assembled head modules. In this method, a test printing is performed to form line segments each extending perpendicular to the main scanning direction, and a distance between the line segments is measured. When the distance differs from a proper distance, actual printing is performed while shifting an image forming position, i.e., impact positions of ink droplets ejected from the head modules, on the recording sheet by an amount corresponding to the difference. In this way, positional shift of the head module with respect to the main scanning direction is cancelled out.
With respect to the secondary scanning direction, line segments perpendicular to the secondary scanning direction are formed, and a distance between the line segments is measured. When the distance differs form a proper one, then the actual positions of the head modules are mechanically moved by a corresponding amount. In this way, positional shift of the head modules with respect to both the main scanning direction and the secondary scanning direction can be adjusted.
There has been also proposed an ink jet recording device including a line-type ink jet head with a wide width formed with a large number of nozzles in one-to-one correspondence with the secondary scanning lines of the recording sheet. In this configuration, there is no need for the print head to scan in the main scanning direction at all, and printing can be performed while continuously feeding the recording sheet in the secondary scanning direction, thereby achieving high printing speed.
In one method, such a line-type ink jet head is produced by forming the nozzles in a straight line at once. However, this method requires a high production cost, because if even one of the nozzles is formed with uneven characteristics, then the overall image quality is degraded.
In order to avoid this problem, there is provided a method to form the head by assembling a plurality of short head modules each formed with nozzles. Because the head modules can be produced in lower cost, overall production cost of the print head can be suppressed. Thus produced print head can perform in the same manner as the print head formed with all nozzles at once.
In this case also, the preciseness in assembly is important but hard to achieve. Japanese Patent-Application Publication No. HEI-9-262992 discloses a method for adjusting the positional shift of the head modules that is inevitable during the assembly. A test pattern is formed by some of the nozzles of each print head or test nozzles provided only for printing the test pattern. Then, the positional shift of the head modules is detected based on the printed test pattern, and the positions of the head modules with respect to the main and secondary scanning directions are mechanically adjusted by using an adjustment mechanism.
Because an electrical manner can be used to adjust the positional shift of the head modules with respect to the main scanning direction, the mechanical manner and the electrical manner can be used in combination.
Japanese Patent-Application Publication No. 2000-127370 discloses a means for determining the positional relationship among the head modules based on printed test patterns. A plurality of test patterns is printed while an adjustment mechanism shifts the positions of the head modules. Then, the color density of the test patterns is detected by an optical sensor. Based on the detected color density, suitable positions of the head modules are determined.
However, because the positional adjustment is performed based on the test patterns that only some of the nozzles of each print head or the test nozzles have printed, characteristic differences among the all nozzles cannot be detected, and so positional adjustment needed for the characteristic differences cannot be adjusted. The characteristics of the nozzles include, for example, flying direction, flying speed, and volume of ink droplet ejected from each nozzle. The nozzle characteristics also vary depending on the ambient environment. Uneven characteristics among the nozzles cause positional shift of printed images and also degrades the image quality.
Also, in order to improve the preciseness in mechanically moving the head modules, the configuration of the adjustment mechanism will be complex. Also, because the adjustment mechanism is required, automatic adjustment is impossible.
Further, the amount of ink that spreads on the recording sheet varies depending on the ambient environment or the type of the recording sheet. The color density also varies depending on the recording sheet. These facts influence on the color density measurement and the like because dots are formed, in the test pattern printing, so close to one another or even overlapped intentionally. Because it is hard to avoid these influences, accurate determination of the positional relationship of the head modules is difficult.
It is an objective of the present invention to overcome the above problems and to provide an ink jet recording device capable of printing high quality images at a high printing speed wherein positional relationship among dot groups of head modules is accurately detected and electrically and automatically adjusted.
In order to overcome the above and other objectives, there is provided an ink jet recording device including at least one head module, deflecting means, a transporting means, a detection means, and a control means. The head module is formed with a row of a plurality of nozzles from which an ink droplet is selectively ejected onto a recording medium. The row extends in a first direction. The deflecting means deflects a flying direction of the ink droplet toward a second direction perpendicular to the first direction by a deflection amount. The ink droplet alights on an optional position of the recording medium. The transporting means transports the recording medium relative to the head modules in a third direction angled from the first direction. The detection means detects a positional error of the optional position with respect to a reference position. The control means controls the deflecting means to change the deflection amount in accordance with the positional error.
There is also provided a controlling method for controlling optional positions of ink droplets ejected from an ink jet recording device. The controlling method includes a) ejecting ink droplets from nozzles arranged in a row extending in a first direction, onto a recording medium transported relative to the nozzles in a second direction angled from the first direction, b) deflecting a flying direction of the ink droplets toward a third direction perpendicular to the first direction by a deflection amount, c) detecting a positional error of optional positions of the ink droplets having alighted on the recording medium with respect to a reference position, and d) controlling the deflection amount in accordance with the positional error.